Electrosuspension, also known as electrodispersion, is a technique for suspending fine particulate matter within closed or open containers and is usually produced by applying high DC-potential to appropriately configured stationary electrodes fixed within a dispersing chamber. The suspension effect is produced by the interaction between the applied electric field and the particles. The suspensions are typically in the form of a dust-cloud which partially fills the container above a static powder bed. Concentration of the cloud may be adjusted by raising or lowering, as required, the voltage that is applied to the electrodes. A typical but not exclusive electrode configuration is one where an electrode is embedded within the static powder bed, while the other is positioned some 20-30 mm above the surface of the powder. Although there are a number of possible polarity combinations which can be used, it is often the case that the embedded electrode is at earth potential. While the applied voltage necessary to cause suspensions is determined by factors such as the relative spacing of the electrodes, the weight, size and shape of particles, it is mostly well above 10 kV and can be as high as 30-40 kV. Particle sizes are typically in the range of from a few microns to several hundred microns.
Developments in the art of electrosuspensions have been reported in the J. of Appl. Phys., 1980, 51 (10 5215-5222 and 5223-5227, and in the J. of Appl. Phys., 1984 55 (11) 4088-4094. Examples of applications of these developments are also given in the United Kingdom patents 2074610B and 2143989.
Prior art electrosuspension apparatus have sufferred several inherent disadvantages:
Firstly, particles easily polarized by the electric field, such as contained by many crystalline dielectrics (e.g.: KCl, NaCl, sugar, ascorbic acid, nicotinamid), tend to align themselves with the field and with each other, forming chains, filaments or needles in the process. There is a tendency for these formations to attach themselves to one of the electrodes and act as field-concentrators, giving rise to intermittent and later continuous ionization of air within the dispersing space. As ionized air is electrically conducting, this mechanism can collapse the high voltage field, resulting in the sharp reduction of the suspended cloud. The formation of filaments can be especially prevalent in case of fibrous dust, such as asbestos and cellulose, and it is often the case that these type of powders form solid bridges extending between the electrodes, while voltage is applied.
Additionally, in applications which involve the treatment or us of the suspended dust, (such as the vapour coating of particles), it is often necessary to remove the suspension from within the electrode space. While removal can sometimes be effected by a cross-airstream through the system, this is not always viable. Removal techniques based on the tendency of particles to `shoot past` the upper electrode, being propelled by their own upward momentum have not generally succeeded, as the fixed upper electrode acts as a physical barrier to the particles. This difficulty has ben addressed by adapting electrode design, for example, by using a wiremesh type configuration. However, the tendency of the particles to eventually block up openings can not be easily eliminated and is particularly prevalent with dielectric dust. A factor further limiting the amount and concentration of dust which can emerge through the electrode region is the reverse charging of particles by physical contact with the electrode, effectively reversing the charge and therefore the direction of force which the particles experience.
Yet another difficulty exists in relation to the electrosuspension of particles having a size of a few microns. Particles of this size are often referred to as micronised particles and as used herein this term refers to particles having a size of less then 30 .mu.m. Hitherto, it has not been possible to effectively generate an electrosuspension of many types of micronised powders.
This has placed severe limitations on the practical use of the electrosuspension process in areas such as the pharmaceutical powders industry, in paint-pigments manufacture and handling, in areas of medical technology and the like where often ultrafine powders must be used with particle sizes in the range of 2-5 .mu.m or less.